The present invention relates to a solvent system and to the use of the solvent system for dissolving one or more curing agents for epoxy resins and/or one or more curing catalysts and/or one or more cure inhibitors.
It is a well known technique to utilize an organic solvent for reducing the viscosity of liquid epoxy resins or liquid curing agents therefore or to solubilize solid resins and/or curing agents in organic solvents in order to facilitate the handling of the epoxy resins and/or of the curing agents.
U.S. Pat. No. 3,679,465 teaches to produce reinforced, hardenable epoxy compositions by continuously passing a reinforcement material through an epoxy solution system, thereby impregnating the reinforcement material with the epoxy solution system and to subject the impregnated reinforcement material to a heat treatment. The epoxy solution system contains a curable epoxy resin, a curing agent therefore and a low boiling solvent. Various types of conventional curing agents or hardeners as well as accelerators are listed, such as primary and secondary amines, amides, polyamines, polyamides, dicyandiamide, benzoguanamine, imidazole, tetramethyl diamine, etc. As useful low boiling organic solvents are mentioned acetone or methyl ethyl ketone, or a mixture of dimethyl formamide, acetone and water.
It is known that mixtures containing an epoxy resin and certain curing agents therefore, such as dicyandiamide, have an excellent storage stability at 20.degree. C. Therefore, these mixtures are useful for producing so-called "one-component systems" which cure upon heating.
East German patent 134 446 teaches that a major disadvantage of known one-component systems is the low solubility of dicyandiamide in the solvents which are used in these systems. The amount of dicyandiamide required for curing the system is often close to or even above its saturation concentration in the solvent. Precipitation of dicyandiamide from the one-component system at low temperature, such as 0.degree. C., is quite frequent. In order to overcome this disadvantage, the East German patent suggests to produce a one-component system by dissolving an epoxy resin and dicyandiamide in a mixture consisting of a glycol ether and water. Water is used in an amount of from 1 to 30 percent, preferably 5 to 15 percent, by the weight of the glycol ether. Useful glycol ethers are said to be ethyl glycol and/or methyl glycol.
WORLD PATENT INDEX, Abstract No. 79-26180B by Derwent Publications Ltd., which abstracts East German patent 133 955, discloses that a hardener for epoxy resins consists of dicyandiamide dissolved in a solvent mixture of glycol ethers, especially ethyl glycol and/or methyl glycol, containing 1 to 50, preferably 5 to 20 wt.-%, water. It is disclosed that the presence of water doubles the dicyandiamide solubility.
Presently, the monomethyl ether and monoethyl ether of ethylene glycol as well as dimethyl formamide are widely used in the industry for dissolving epoxy hardeners like dicyandiamide.
However, the wide use of ethylene glycol monomethyl ether, ethylene glycol monoethyl ether and dimethyl formamide has raised some concern among environmentalists.
It is known from Swiss patent 257,115 to heat an epoxy resin in the presence of a solvent with dicyandiamide and an aldehyde condensation product containing etherified methylol groups, such as alkyl ethers of methylolmelamine or of methylolureas. It is suggested to dissolve the epoxy resin in an organic solvent, such as 2-methyl-pentanediol-(2,4), toluene, cyclohexanol, etc. and to add the other components. Alternatively, the epoxy resin can be combined with a mixture of dicyandiamide, aldehyde condensation product and solvent. From 2 to 20 percent, preferably from 6 to 10 percent, of dicyandiamide is used for curing, based on the weight of the epoxy resin. The Swiss patent teaches that the amount of the aldehyde condensation product must be high enough to enable a complete absorption of dicyandiamide in the resin solution. Lacquer resin solutions are produced which can be used for applying coats of lacquer on metals. These coats of lacquer have good properties, such as good adherence to metals and high resistance to chemicals.
WORLD PATENT INDEX LATEST, Abstract No. 91-003223 by Derwent Publications Limited, abstracting published Japanese patent application JP-A-2,279,776 discloses an ink composite containing an epoxy resin, a hardening agent and a solvent. Dicyandiamide, imidazole compounds, triazine compounds, urea compounds, aromatic amine compounds and one or more kinds of photo-cationic polymerization catalysts are used as a hardening agent. The compound of formula R.sub.1 --(OR.sup.2).sub.n OR.sup.3 and one or more kinds of solvent naphtha are used as a solvent. R.sup.1 is hydrogen or C.sub.1-8 -alkyl, R.sup.2 is methylene or ethylene, R.sup.3 is hydrogen or --C(O)--R.sup.4 wherein R.sup.4 is C.sub.1-8 -alkyl and n is 1 to 4.
Published German patent applications DE-A-2,545,149 and DE-A-2,650,408 suggest to utilize a mixture of diacetone alcohol and water for dissolving dicyandiamide. It is recommended to dissolve 3 weight parts of dicyandiamide in 65 weight parts of diacetone alcohol and 13.3 weight parts of water. However, trials made by the Applicants have shown that dicyandiamide dissolves very slowly in the suggested mixture of diacetone alcohol and water. Apparently no more than 3 weight parts of dicyandiamide can be dissolved in the suggested solvent mixture. Furthermore, upon addition of an epoxy resin solution described in detail in the Examples further below, the dicyandiamide solution in diacetone alcohol and water turns turbid. The turbidity is an indication that dicyandiamide crystallizes.
Accordingly, one object of the present inventions is to find a new solvent system. Another object of the present invention is to find a solvent system which is useful for dissolving a curing agent for an epoxy resin, for dissolving a curing catalyst or a cure inhibitor or for dissolving a blend thereof.